1. Field of the Invention
The present invention relates to a semiconductor porcelain composition for use in a PTC thermistor, a PTC heater, a PTC switch, a temperature detector or the like, which has a positive temperature coefficient of resistivity; and a method for producing the same.
2. Description of the Related Art
Conventionally, as materials showing a PTCR characteristic (Positive Temperature Coefficient of Resistivity), there have been proposed compositions in which various semiconductive dopants are added to BaTiO3. These compositions have Curie temperatures around 120° C. Depending on the use, these composition need to be shifted in the Curie temperatures.
For example, although it has been proposed to shift Curie temperature by adding SrTiO3 to BaTiO3, in this case, the Curie temperature is shifted only in a negative direction and is not shifted in a positive direction. Currently, only PbTiO3 is known as an adding element for shifting Curie temperature in a positive direction. However, PbTiO3 includes an element for bringing about environmental pollution, and therefore, in recent years, a material which does not use PbTiO3 has been desired.
In a BaTiO3 semiconductor porcelain, with an object of preventing a reduction in a temperature coefficient of resistivity by Pb substitution, as well as reducing a voltage dependency and promoting a productivity or a reliability, there has been proposed a method of producing a BaTiO3 semiconductor porcelain in which a composition, not using PbTiO3 and obtained by adding one or more kinds of any of Nb, Ta and a rare earth element to a composition in which x in Ba1-2x (BiNa)xTiO3 where a portion of Ba of BaTiO3 is substituted by Bi—Na is controlled to a range of 0<x≦0.15, is sintered in nitrogen, and thereafter subjected to a heat treatment in a reducing atmosphere (see Patent Reference 1).                Patent Reference 1: JP-A-56-169301        